In the field of color image processing, it is sometimes desired to apply the processing only to a certain region of interest, such as cutting out or color change of the object. Such requirement is strong particularly in the color image editing systems and painting systems which have rapidly been commercialized in personal computers or work stations. These are greatly different from the image processing in remote sensing systems or medical systems in which mainly the whole image is subject to processing, in that only a required portion needs to be processed. In such partial processing, it will be important in terms of processing efficiency and memory efficiency that a required portion can promptly be detected and the region can be read or written with as little loss in efficiency as possible. Accordingly, the data structure also needs to be suitable for such partial processing. This is particularly essential when the system is put into practical use for cases in which many objective regions exist in a still image, or many continuous frames are processed as in a moving image.
Color image data is typically stored as data of the three primary colors for each picture element in a two-dimensional array. In this case, as the value of a picture element is basically described as a function of position, in order to detect the region to be processed, it is required to read out the whole image and check whether the condition is fulfilled for each picture element.
Incidentally, as an effective method for handling graphical information and image data, there are a tree structure and a pyramid structure. For instance, if the data of a binary image is expressed by the tree structure, the region is more finely divided where information exists more densely, so that the image data can be efficiently managed and it is possible to fast retrieve whether a white or black picture element exists at a specified position. However, conversely, if an objective region, for instance, a region of white picture elements, is desired to be retrieved, all the nodes of the lowest level must eventually be examined. If information is very dense, the lowest level could be the picture element level. In this regard, the retrieval efficiency decreases because all the lowest level nodes must be traversed. Although the tree structure can also be applied to multivalued image data, is it similar in this respect.
On the other hand, the pyramid structure is effective when an image of lower resolution is processed and then an image of higher resolution is processed. However, because some information may be naturally lost in such an image of low resolution, this structure is not suitable for, for example, processing in which all the regions of white picture elements are to be retrieved.
The prior arts of the color processing of color image data include Japanese Patent Application Laid-Open No. 182786/1988 disclosing a technique for determination of edges in a specified objective region, Japanese Patent Application Laid-Open No. 184475/1988 disclosing a technique for selection of a representative color for dividing an objective region, and Japanese Patent Application Laid-Open No. 185163/1988 disclosing a technique for utilizing a color specification system other than the RGB color specification system in order to increase the color conversion, lightness conversion and data compression rates. However, these prior arts do not suit retrieval of the region having a specified color, nor disclose a data management method therefor.
As described above, it is the problem of the prior art to enable a region having a specified color to be fast retrieved in a color image. Accordingly, it is the object of the present invention to provide a color image region management method suitable for retrieving a region having a specified color, and a retrieval method for fast retrieving the region having a specified color by the use of the color image management method.